The present invention relates to electroluminescent devices. Examples of electroluminescent devices include organic light emitting devices (OLED), polymer light emitting devices (PLED), and inorganic electroluminescent devices.
A typical prior art electroluminescent device comprises a transparent substrate, a transparent first electrode layer, a light-emitting element including at least one light-emitting layer, and a reflecting second electrode layer. Light is generated in the electroluminescent device when electrons and holes that are injected from the two electrodes flowing through the light-emitting element and generating light by either recombination or impact ionization. The light-emitting element can include several layers of materials including at least a light-emitting layer where the emitted light is generated. In the case of an OLED device, for example, the light-emitting element can include an electron injection layer, an electron transport layer, one or more light-emitting layers, a hole transport layers, and a hole injection layer. One or several of these layers can be combined or eliminated and additional layers such as electron or hole blocking layers can be added. Most frequently, the first electrode layer is the anode and the second electrode layer is the cathode.
The light-emitting material has an index of refraction larger than that of the air and most frequently there is also one or more layers between the light emitting layer and air having index of refraction smaller than that of the light-emitting layer but larger than that of air. As the light travels from a higher index layer into a lower index layer total internal reflection can take place, the totally internal reflected light cannot transmit into the lower index layer and is trapped in the higher index layer. In the case of an OLED device, for example, the light emitting layer typically has an index of refraction of 1.7 to 1.8; the transparent electrode layer has an index of about 1.9, and the substrate has an index of about 1.5. Total internal reflection can take place at the transparent electrode/substrate interface. The fraction of the light from the light-emitting layer arriving at this interface with larger than critical angle from the normal is trapped within the organic layers and the transparent electrode layer and eventually absorbed by the materials in these layers or exited at the edges of the OLED device serving no useful functions. This fraction of light has been referred to as the organic-mode of light. Similarly, total internal reflection can take place at the substrate/air interface. The fraction of light arriving at this interface with larger than critical angle from the normal is trapped within the substrate, the transparent electrode layer, and the organic layers and eventually absorbed by the materials in the device or exited at the edges of the OLED device serving no useful function. This fraction of light has been referred to as the substrate-mode of light. It has been estimated that more than 50% of light generated by the light-emitting layer ends up as the organic mode of light, more than 30% ends up as the substrate mode of light, and less than 20% of light from the light-emitting layer can actually be outputted into the air and become useful light. The 20% of generated light that actually emits from the device has been referred to as the air-mode of light. Light trapping due to total internal reflection thus decreases drastically the output efficiency of electroluminescent devices.
Various techniques have been suggested to increase the efficiency of the thin-film electroluminescent devices by reducing the light trapping effect and allow the substrate-mode and organic-mode of light to emit from the device. These attempts are described in the references in detail and are included here by reference: U.S. Pat. Nos. 5,955,837, 5,834,893; 6,091,195; 6,787,796, 6,777,871; U.S. Patent Application Publication Nos. 2004/0217702 A1, 2005/0018431 A1, 2001/0026124 A1; WO 02/37580 A1, WO02/37568 A1.
In general, these attempts all provide an enhancement structure that can change the direction of light such that some of the light that would have been trapped because of total internal reflection can emit into the air. Most of the enhancement structures, however, are placed on the outside surface of the transparent substrate opposite to the surface where the electroluminescent device is disposed. These enhancement structures can only access the air-mode light and the substrate-mode light since the organic-mode of light never reaches these structures. Since the organic-mode light constitute about half of the light generated, these enhancement structures are not very effective in enhancing the output of the electroluminescence device. To effectively improve the extraction of all three modes of light, the enhancement structure has to be placed close to the transparent electrode. For a bottom emitting structure that the present invention relates to, placing the enhancement structure close to the transparent electrode means the enhancement structure has to be placed inside the electroluminescent device between the transparent electrode and the substrate. Constructing this internal enhancement structure presents difficult technical challenges, however, since thin-film electroluminescent devices are very delicate. Placing the enhancement structure inside the device structure can cause many undesirable consequences including totally shorting out the devices. Although there have been suggestions of internal enhancement structures, no practical device structure have been described in the prior art that resulted in effective enhancement of light extraction efficiency.